The invention relates to an armoured cable having mineral insulation, comprising a metal conductor and a hollow metal cylindrical sheath surrounding the conductor and between which a powdered refractory insulating material is interposed.
The invention is employed in electronic measuring and recording systems for transmitting electric signals supplied by sensing means for physical quantities.
Such a cable is known from, for example, European Patent Application No. 0,084,171 AL. This application describes an electric cable comprising a central titanium conductor and a hollow metal sheath also of titanium, the central conductor being insulated from the sheath by a mineral refractory insulating material, such as magnesium oxide or aluminum oxide.
Although this European Patent application notes the manufacturing problems which are attributable to the metal used for the conductor and the sheath, it does not solve the problems which would arise if an insulating material other than magnesium oxide or aluminum oxide were used.
Consequently, one of the essential conditions for the cables to perform well is that these cables, which may be very long (&gt;20 m), are homogeneous over their entire length. This condition is fulfilled only if the mineral insulation, which is introduced in powder form between the central conductor and the sheath during the production process, flows well and spreads evenly over the entire length of the cable, and if the grains of the powder are homogeneous. This is the reason why so far only magnesium oxide or aluminum oxide has been used as an insulating material.
In order to adapt these cables to two types of devices to which they are connected, that is to say, on the one hand to sensing means and on the other to measuring and recording systems, the materials forming the cable must meet certain requirements.
Firstly, they must be resistant to high temperatures. In the prior art this problem is solved by using a metal conductor and a metal sheath and by employing a refractory insulating material such as magnesium oxide or aluminum oxide.
Moreover, they should have characteristic impedances of a predetermined value, for example, 30, 50 or 75.OMEGA..
It is very difficult to manufacture cables having characteristic impedances which are so high, when use is made of the refractory insulating material described in this Patent Application. As a matter of fact, the characteristic impedance of a cable is an inverse function of the square root of the dielectric constant of the insulation as well as of the diameter of the central conductor. If refractory insulating materials having a high dielectric constant are used, such as magnesium oxide or aluminum oxide, it is necessary, in order to obtain the characteristic impendance within a predetermined range, to use a central conductor of a very small diameter, thus rendering the manufacture difficult and costly.
It has also been proposed to use very pure SiO.sub.2 powder comprising at least 98% by weight of SiO.sub.2 as a mineral insulating material. Cables having such an insulation are used at hyperfrequencies instead of at the usual high frequencies. However, cables having an SiO.sub.2 insulation are difficult to manufacture. An extrusion process is used for their manufacture, in which a central wire which is coated with a layer of molten SiO.sub.2 is elongated until the desired diameter of the wire and the coating is obtained, The SiO.sub.2 -coated central conductor is led into a hollow tube which is also subjected to a drawing process until the desired diameter is obtained. These process steps are very laborious. The resultant cable has a high porosity of approximately 60% (percentage of air within the outer conductor). Consequently, the central conductor is poorly positioned and may break easily.